The map and list lookup caches had twin entry classes and twin put and
prune helpers that differed only in the value type. Replace them with
_BridgeCacheEntry<T>, _putMemoryCached<T> and _pruneExpiredBridgeCache<T>.
Each cache keeps its own map, TTL and budget accounting; the byte estimate,
shared budget constants and oldest-first eviction are unchanged.
Top-level helpers and secondary classes (codec isolates, cache entries,
event and SAF models, the native worker job state and the NDJSON scan
file) move verbatim into part files grouped by concern. The
PlatformBridge class body is unchanged apart from three region comments
over groups that were already contiguous.
platform_bridge.dart was one 2900-line file mixing eight concerns; the
static class cannot be split without changing its type, so this moves
what can move mechanically and leaves the class intact for call sites
and the channel mocks in tests.
zarzet